Variable impedance transmission line



June 6, 1967 H. BRUECKMANN 3,324,426

VARIABLE IMPEDANCE-TRANSMISSION LINE Filed Oct. 8, 1964 2 Sheets-Sheet 1 FIG.5

MIDPOINT RIGHTEND DISTANCE FROM LEFTEND OF SLUG INVENTOR, HELMUT BRUEGKMANN June 6, 1967 Filed Oct. 8, 1964 'sTAcK 5 STACK H. BRUECKMANN 3,324,426

VARIABLE IMPEDANCE TRANSMISSION LINE 2 Sheets-Sheet 2 FIGS I l I I I L0 L5 2.0 THICKNESS RATIO CERAMIC/FERRITE FIG.7

THICKNESS RATIO CERAMIC/ FERRITE W LWZLM C.

INVENTOR, HELMUT BRUECKMANN ATTORNEYS United States Patent O 3 324,426 VARIABLE IMPEDAhICE TRANSMISSION LINE Helmut Brueckmann, Little Silver, N.J., assignor to the United States of America as represented by the Secretary of the Army Filed Get. 8, 1964, Ser. No. 492,669 6 tllaims. (Cl. 33334) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to transmission lines and more particularly to transmission lines characterized by a controllable characteristic impedance.

In antenna arrays, it is well known that the current of the elements in the array must be adjusted in phase and amplitude for a particular relationship in order to obtain a particular radiation pattern. In such systems it is desirable that the control for phase and amplitude of the current of the elements in the array vbe independent of each other. One system of gradually adjusting and controlling the amplitude ratios without changing phase is shown and described in US. Patent No. 2,938,209, May 29, 1960. Also in my copending application Ser. No. 315,- 093, now Patent Number 3,219,950, there is shown a system which, by means of a low loss variable delay line, resolves only the problem of controlling the phase of an element without changing the amplitude or impedance ratio in the device itself. While such singular control systems and other similar systems operate satisfactorily, they have proved to be rather cumbersome, inefiicient, and difiicult to adjust.

It is an object of the present invention to provide an improved transmission line wherein the aforementioned limitations are overcome.

It is another object of the present invention to provide an improved transmission line which permits gradual control and adjustment of the input-output amplitude relationship of an RF signal propagated thereon.

In brief, there is provided a variable impedance trans mission line which includes a pair of electrical conductors and a plurality of successively arranged elements intermediate the conductors which are adapted to be slideably positioned with respect to said conductors. Each of the elements comprise alternate slices or rings of a ferrite material and a dielectric material, preferably a ceramic having a high dielectric constant. The parameters of the ferrite and dielectric materials comprising each of the successively arranged elements are such that each of said elements are characterized by a discrete effective relative permeability, ,u and a discrete effective relative dielectric constant, t the output product (fl ff'g ff) of each of said elements being equal and the ratio of n /E of each of said elements changing monotonically from one end of the successively arranged elements to the other end thereof.

For a better understanding of the invention, together with other and further objects thereof, reference is bad to the following description taken in connection with the accompanying drawing in which:

FIGS. 1 and 2 are side-sectional and end views, respectively, of a variable impedance transmission line in accordance with the teaching of the invention, with FIG. 2 being a sectional view along the lines 2-2 of FIG. 1;

FIGS. 3 and 4 are side-sectional and end views, respectively, of another embodiment of the invention with FIG. 4 being a sectional view along the lines 4-4 of FIG. 3; and

FIGS. 5, 6 and 7 are explanatory curves.

It is well known that the propagation velocity of a co axial type transmission line having a medium between conductors characterized by an effective relative permeability a and a permittivity g is not changed if this medium is replaced by another medium of different relative permeability, ,u', and the effective relative dielectric or permittivity, 5', provided the product ([LX) equals the product txg). The characteristic impedance, however, is lowered or raised by such a replacement since it is proportional to the square root of the ratio of the elfective relative permeability and permittivity for a given coaxial configuration. For the coaxial line having an eifective relative permeability n and a permit-tivity 5, the characteristic impedance is given 'by where and 5 are the permeability and permittivity of free space, respectively, and D and d represent the outer and inner conductor diameters, respectively. Similarly, for the coaxial line having an effective relative permeability ,u. and a permittivity 5', the characteristic impedance may be given by Referring now to FIGS. 1 and 2 of the drawing, there is shown at 10 a coaxial transmission line section having an outer conductor 12 and an inner conductor 14. The inner conductor 14 is supported and held fixed at its ends by a pair of longitudinally spaced, parallel arranged, radial conductors 16 and 18 which extend through and are afl'ixed to insulator plugs 20 and 22 respectively provided therefor. As shown, insulator plugs 20 and 22 are aflixed to outer conductor 12 to form an integral part thereof. Successively arranged axially disposed tubular elements 24 and 26, each comprised of alternately disposed rings or slices of a ferrite material, F, and a ceramic material, C, having a high dielectric constant, are concentrically positioned with respect to the outer and inner conductors 12 and 14 of coaxial transmission section 10. The length of each of the tubular elements 24 and 26 are made as long as or slightly longer than the inner conductor 14. As shown, the tubular elements 24 and 26 are spaced from the conductors 12 and 14 so that a radial air space exists between the tubular elements 24 and 26 and inner conductor 14, and also between the tubular elements 24 and 26 and outer conductor 12. Tubular elernents 24 and 26 are provided with aligned longitudinal slots 28 and 30 having a width slightly larger than the width of radial conductors 16 and 18 so that the tubular elements 24 and 26 may be longitudinally actuated along the axis of the transmission line for the entire lengths thereof. Although not limited thereto the surfaces 'bounding longitudinal slots 28 and 30 are shown in FIG. 1 as being radial and include respective angles for tubular element 24, and 0 for tubular element 26, therebetween. With the longitudinal slots 28 and 30 aligned with the radial conductors 16 and 18, the tubular elements 24 and 26 may be slideably positioned along the longitudinal axis of the coaxial line section 10 for the entire respective lengths thereof. Of course, any suitable means well known in the art may be used to slideably position tubular elements 24 and 26 longitudinally intermediate the outer and inner conductors of coaxial line 10* as described above. The effective relative permeability r and the permittivity g of tubular elements 24 and 26 are functions of the ratio of the thicknesses of the ceramic and ferrite rings or slices. The effective permeability and permittivity of the tubular elements 24 and 26 are simply the weighted average of the respective values of the two materials, the weighing factor being the thickness ratio of ferrite and ceramic. In mathematical language, the values of ,u and g of tubular elements 24 and 26 may be expressed as am (stacked) i Ferrite+P] and g. (stackcd)= [(g Ferrite+ (Een) ceramic] where P is the ratio of the thicknesses of the ceramic and ferrite slices. FIGS. 6 and 7 illustrate the relationship of Equations 3 and 4, respectively, for a typical stacked element having a given diameter and a given slot width. The values of he and E for tubular element 24 are chosen to give a desired characteristic impedance K in accordance with Equation 1. Tubular element 26 is identi cal in structure to tubular element 24, with values ,u eff and differing from that of tubular element 24 but with the Same Product 50 that (H'errXE'err)=(l-err err)- The characteristic impedance K of course will be in accordance with Equation 2. By this arrangement, tubular element 26 may be inserted in place of tubular element 24 to provide changes in characteristic impedance of the coaxial line but not in its electrical length. In actual operation, radial conductor 18 may terminate into a load such as one element of an antenna array. As a result, the input impedance of the coaxial line 10 seen by a signal generator at radial conductor 16 changes in accordance with well known laws when tubular element 24 is replaced by tubular element 26. Of course the input impedance and the change in impedance depends on the ratio of the load impedance and the characteristic impedances K and K respectively, as well as the electrical length of the line. While only two tubular elements are shown in FIG. 1, it is to be understood that the invention is not to be limited thereto. A suflicient number of such successively arranged tubular elements may be utilized to provide sufficiently small increments of associated characteristic impedances so that any variation of the input impedance may be achieved. In this case, the ratio r /g of each of the elements is such as to change monotonically from one end of the successively arranged elements to the other end thereof. In order for the coaxial line 10 to he considered as one having distributed rather than lumped parameters, the thickness of a pair of ferrite and ceramic rings is made small compared to the wavelength of the highest operating frequency.

FIGS. 3 and 4 illustrate another embodiment of the invention where like numerals refer to like elements. Referring now to FIGS. 3 and 4, there is shown at 10 a coaxial transmission line section having an outer conductor 12 and an inner conductor 14. The inner conductor 14 is supported and held fixed at its ends by a pair of longitudinally spaced, parallel arranged radial conductors 16 and 18 which extend through and are afiixed to insulator plugs 20 and 22, respectively, provided therefor. As shown, insulator plugs 20 and 22 are affixed to outer conductor 12 to form an integral part thereof. Longitudinally positioned within coaxial line 10 and concentrically disposed with respect to the outer and inner conductors 12 and 14 is a tubular element 32 of about twice the length of inner conductor 14. Tubular element 32 is comprised of alternately disposed rings or slices of a ferrite material, F, and a ceramic material, C, having a relatively high dielectric constant. As shown, one-half of tubular element 32, as at 34, is of uniform cross-section with the ratio being a constant value. The other half of tubular member 32, as at 36, is comprised of alternate rings of a ferrite material, F, and a ceramic material of high dielectric constant, C, such that the ratio /g of a pair of rings, that is, a ferrite ring and an adjacent ceramic ring, changes monotonically in a geometric progression from the extreme left end of tubular member 32 to the longitudinal center thereof. These changes may be affected by changes either of diameters, slotwidth, or thickness ratio, or a combination thereof. If the rate of change of the ratio /g is made very small from one pair of rings to the next, the outer periphery of section 36 may taper exponentially from the left end thereof to the longitudinal center of tubular element 32. FIG. 5 graphically illustrates the value of the ,u /g ratio from one end to the other. It is well known that an exponential line has the property of transforming at frequencies well above cut-off a matched terminating impedance into a resistive input impedance of different magnitude. As in FIG. 1, the tubular element 32 is provided with a longitudinal slot 38 having a width slightly larger than the width of radial conductors 16 and 18 so that tubular element 32 may be longitudinally actuated along the axis of transmission line 10. Although not limited thereto, the surfaces bounding longitudinal slot 38 may 'be radially disposed. The transfer ratio of the line shown in FIG. 3 depends primarily on the rate of change of the characteristic impedance and the length of the transmission line. While such a line has an imaginary component, which depends somewhat on frequency, under certain well known conditions this imaginary component can be made to be small or can be compensated for. Such conditions can be met over a wide frequency range by proper choice of line length and rate of change of characteristic impedance. It is to be understood, of course, that the invention is not to be limited to the exponential configuration shown in FIG. 3. For example, the tapered half of the tubular element 32 may comprise a taper which alternates several times between upward or downward exponential taper and zero rate of taper. Also, the exponential taper may be the reverse of that shown in FIG. 5. The degree of the taper, of course, may be achieved by increasing or decreasing the increment /5 ratio of successive pairs of rings as described above.

In discussion the operation of the transmission line shown in FIG. 3, assume first that the right end of the line is terminated into a load which matches the line when tubular member 32 is in its extreme left position. In this position the transformation ratio is 1:1. When the tubular member 32 is in its extreme right position, the transformation ratio is highest or lowest, depending on whether the characteristic impedance tapers up or down, respectively, from the termination towards the input end. Any value of the transformation ratio between these extremes can also be obtained within the limits given by the chosen incremental change of l /5 from one pair of rings to V the next. This gradually controllable transformation ratio is achieved. without changing the phase relation between the input and output signal and without any power loss other than that caused by the loss factor of the materials comprising tubular member 32 and the conductor resistivity. Ina practical embodiment, all dimensions can be kept constant except the thickness ceramic/ferrite ratio by utilizing the relatively level portion of the [,u (stackedyg (stacked)] of FIG. 6 about its maximum. While efi (stacked) Sen (stackedfl is nearly constant, the ratio of a (stacked)/ (stacked) as shown in FIG. 7, changes rapidly in the region for P=0.4 to P=l.0.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention. For example, line configurations other than the coaxial line configurations such as strip line and balanced open wire line may be used. As another example, two electrically separate lines in one unit, back-to-back for example, may employ a single tubular element or a differential transformer.

What is claimed is:

1. A transmission line transformer comprising a pair of electrical conductors, a plurality of successively arranged elements intermediate said conductors, each of said elements comprising alternate slices of a ferrite material and a dielectric material having parameters such that each of said elements are characterized by a discrete effective relative permeability heft and a discrete effective relative dielectric constant, g the product of the effective relative permeability and the effective relative dielectric constant of each of said elements being equal, and the ratio of n /5 of each of said elements changing monotonically from one end of successively arranged elements to the other end thereof to provide a monotonically varying characteristic impedance, said elements being slidably positioned with respect to said conductors.

2. A transmission line transformer comprising an inner conductor and outer conductor coaxial therewith, a plurality of successively arranged tubular elements adapted to be axially positioned intermediate said conductors, each of said tubular elements comprising alternate rings of a ferrite material and a dielectric material having parameters such that each of said tubular elements are characterized by a discrete effective relative permeability pm and a discrete effective relative dielectric constant s the product of the effective relative permeability and the efective dielectric constant of each of said tubular elements being equal, and the ratio heft/56ft of each of said elements changing monotonically from one end of the successively arranged elements to the other end thereof to provide a monotonically varying characteristic impedance.

3. The transmission line in accordance with claim 2 wherein the tubular elements are spaced from both said inner and outer conductors when intermediate said conductors.

4. The transmission line in accordance with claim 2 wherein each of said tubular elements are as long as said center conductor.

5. In combination with a coaxial transmission line adapted to propagate an RF signal, means for simultaneously varying the characteristic impedance of said c0- axial transmission line and maintaining a constant propagation velocity, said means comprising a plurality of successively arranged tubular elements adapted to be axially positioned intermediate the inner and outer conductor of said coaxial line, each of said tubular elements comprising alternate rings of a ferrite material and a dielectric material having parameters such that each of said tubular elements are characterized by a discrete effective relative permeability neg and a discrete effective relative dielectric constant g the product of the effective relative permeability and the effective relative dielectric constant of each of said tubular elements being equal, the ratio of /g of each of said elements changing monotonically from one end of the successively arranged tubular elements to the other end thereof, said successive tubular elements being adapted to be axially positioned intermediate said conductors.

6. A transmission line for propagating an RF signal comprising, an inner conductor and an outer conductor coaxial therewith, a tubular member adapted to be axially positioned intermediate said conductors, one longitudinal half of said tubular member being uniform in cross section and comprising alternate rings of a ferrite material and a dielectric material characterized by a constant value of the ratio of the effective relative permeability and the effective relative dielectric constant, the other longitudinal half of said tubular member comprising alternate rings of a ferrite material and a dielectric material having parameters such that the ratio of the effective relative permeability and the effective relative dielectric constant of any one pair of alternate rings is a discrete value and changing monotonically along said other longitudinal half of said tubular member from one end thereof to the other end thereof.

References Cited UNITED STATES PATENTS 2,877,433 3/1959 Devot 33373 2,911,554 11/1959 Kompfner 3153.5 3,125,733 3/1964 Holembeck 333-79 3,191,132 6/1965 Mayer 333-79 3,200,355 8/1965 Dablen 333-79 3,219,950 11/1965 Brueckmann 33331 ELI LIEBERMAN, Primary Examiner.

HERMAN KARL SAALBACH, Examiner.

C. BARAFF, Assistant Examiner. 

1. A TRANSMISSION LINE TRANSFORMER COMPRISING A PAIR OF ELECTRICAL CONDUCTORS, A PLURALITY OF SUCCESSIVELY ARRANGED ELEMENTS INTERMEDIATE SAID CONDUCTORS, EACH OF SAID ELEMENTS COMPRISING ALTERNATE SLICES OF A FERRITE MATERIAL AND A DIELECTRIC MATERIAL HAVING PARAMETERS SUCH THAT EACH OF SAID ELEMENTS ARE CHARACTERIZED BY A DISCRETE EFFECTIVE RELATIVE PERMEABILITY UEFF AND A DISCRETE EFFECTIVE RELATIVE DIELECTRIC CONSTANT, $EFF, THE PRODUCT OF THE EFFECTIVE RELATIVE PERMEABILITY AND THE EFFECTIVE RELATIVE DIELECTRIC CONSTANT OF EACH OF SAID ELEMENTS BEING EQUAL, AND THE RATIO OF UEFF/$EFF OF EACH OF SAID ELEMENTS CHANGING MONOTONICALLY FROM ONE END OF SUCCESSIVELY ARRANGED ELEMENTS TO THE OTHER END THEREOF TO PROVIDE A MONOTONICALLY VARYING CHARACTERISTIC IMPEDANCE, SAID ELEMENTS BEING SLIDABLY POSITIONED WITH RESPECT TO SAID CONDUCTORS. 